Recently, in practicing medical radiography, it is required to acquire and hold the exposure dose or absorbed dose of a patient as an area dose.
In order to meet this requirement, a method is used, in which an exit dosimeter (also called an area dosimeter) is placed near an X-ray source to measure an area radiation dose, and the measurement value is used as an area exposure dose. With this method, the area exposure dose of each patient is managed by a database.
It is, however, preferable not to use any exit dosimeter because it increases the cost of a radiography system. In accordance with such a market demand, a technique of predicting an area radiation dose from emission conditions (a tube current, tube voltage, emission time, and the like) and using the predicted dose as an area exposure dose has also been proposed. This method is disclosed in Japanese Patent No. 3133741.
On the other hand, recently, a digital radiography system has been increasingly used, in which elements for outputting electric charges in accordance with the amounts of X-rays received are arranged on a plane, and X-rays are captured as digital values to form a plane image. As disclosed in Japanese Patent Laid-Open Nos. 11-272851, 2001-144932, and 2001-144966, a digital radiography system can also recognize a region irradiated with X-rays (to be referred to as an “irradiation region” hereinafter) by scanning a captured image. X-rays transmitted through the human body become weaker than those which are not transmitted through the human body. This makes it possible to discriminate and recognize a region where X-rays are transmitted through the human body (to be referred to as an “object region” hereinafter) and a region where the sensor is directly irradiated with X-rays (to be referred to as a “non-object region” hereinafter). This technique is also disclosed in the above references. FIG. 12 shows a conceptual view of an irradiation region, object region, and non-object regions.
Japanese Patent Laid-Open No. 10-52421 discloses a method of measuring the absorbed dose of a patient by using both an exit dosimeter and a digital radiography system. This method allows an operator to grasp the absorbed dose of a patient during radiography and can give a warning before irradiating the patient with excessive radiation.
In both the measuring method using a conventional exit dosimeter and the method of predicting a radiation dose from emission conditions and using the predicted value as the exposure dose of a patient, all the emitted X-rays are assumed to be transmitted through the human body, and are used as an exposure dose. In actual radiography, however, a region where no X-rays are transmitted through the human body (non-object region) may exist in even an irradiation region. In the presence of such a non-object region, the area radiation dose becomes higher than the actual area exposure dose of the human body. It is therefore inappropriate to use the area radiation dose as an area exposure dose without any change. Assume that a specified value of an area exposure dose is set, and radiography is inhibited above the specified value. In this case, since the held value exceeds the actual exposure dose, necessary radiography cannot be performed.
In radiographing the chest, in particular, since an X-ray image usually includes non-object regions, it is an urgent requirement to solve the above problem.
In the method of measuring the absorbed dose of a patient by using both a conventional exit dosimeter and a digital radiography system, two calibrations are required, including a calibration for the exit dosimeter and a calibration for the digital radiography system.